Considering
the cost of putting to sea, it is well worth investing in vibration
monitoring equipment that can help engineers to protect and maximise
the performance of marine machinery. The challenges facing engineers
and advice on how to successfully install and use vibration sensing
equipment on marine vessels.

As machinery
has become more complex and mankind has demanded greater productivity
from each mechanical system, there has been a corresponding growth
in the need for sophisticated vibration sensors that can help
maximise the performance of many engineering processes. This
is especially true at sea, where vibration monitoring is a key
tool used by marine engineers to manage the availability and
maintenance of a wide range of equipment with rotating shafts,
including pumps, motors, fans, gearboxes and engine systems.

The cost of putting
an oil tanker or a cruise ship to sea has always justified the
cost of monitoring it but in recent times pressure has grown
on ship owners to ensure that they meet deadlines. Add to this
the fact that much bad publicity can be generated by severe failures,
whether caused by the unscheduled docking for repairs of a container
ship or the malfunction of ventilation fans in hotels on cruise
ships, and the need to apply reliable preventative maintenance
techniques becomes ever-more pressing. Thankfully, todays
designers and engineers have succeeded in developing a range
of tools and practices to prevent vibration and its consequences,
such as alignment tools and automatic lubricators that can be
applied during system construction, while components themselves
are continually being refined and upgraded to offer greater resistance
to vibration. However, vibration can never be entirely banished
from rotating machinery and so there has been much development
in vibration monitoring technology; in particular, vibration
sensors increasingly offer exceptional reliability packaged in
a variety of resilient enclosures to enable their use within
a wide range of applications.

The adoption
of vibration monitoring equipment is vital in marine applications
because vibration is one of the main causes of failure in marine
propulsion systems and auxiliary equipment. The development of
vibration monitoring equipment has naturally involved examining
the key causes of vibration, such as poor alignment of rotating
shafts in, for example, propulsion systems and turbochargers
for main or auxiliary engines.

Vibration monitoring
offers a vital early warning, enabling engineers to take action
before any substantial damage is caused. When machinery such
as fans and motors run out of alignment, the resulting vibration
leads to excessive wear and premature failure of parts and, ultimately,
a resulting reduction in efficiency. Gaining access to motors
and fan units can be difficult and time consuming and without
a strong condition monitoring regime engineers will find themselves
continually reacting to problems that have already been created.

A good illustration
of this is to consider how vessels have coped in recent years
without condition monitoring. Take, for example, a tanker that
is subject to inspection by a class surveyor, who makes an annual
check on the equipment. Without condition monitoring, the only
way to analyse equipment is for on-board engineers to take it
apart and show the pieces to the surveyor, which naturally consumes
much valuable time and manpower and can only be done when the
vessel is in dock. On large vessels, the surveyor may not be
able to inspect more than, say, 20% of equipment every year,
meaning that it might be five years between examinations for
some machine parts. That leaves many machine parts unexamined
for long periods of time and even those parts that are examined
can suffer because taking machinery apart and reassembling can
cause its own problems. Many marine engineers have observed that
a large percentage of defects are maintenance induced, so the
less they have to take things apart and put them back together,
the better. In contrast, condition monitoring, such as vibration
monitoring on pumps, motors, fans, gearboxes and engine systems,
allows marine engineers to maintain equipment far more efficiently
with less manpower and maintenance.

The above example
explains why vibration monitoring of marine machinery, especially
propulsion and manoeuvring systems, engines and turbochargers,
is becoming a more widely adopted facet of condition monitoring
that, alongside other powerful tool such as oil monitoring and
thermal imaging, is protecting profits and enhancing performance.
To appreciate how vibration monitoring is bringing these benefits
to marine applications, lets look at the current technology.

The current array
of sensors or accelerometers for vibration monitoring offered
by market leaders such as Hansford Sensors can operate over a
wide temperature range, measuring both high and low frequencies
with low hysteresis characteristics and excellent levels of accuracy.
These devices also offer robust and reliable service, thanks
to stainless steel sensor housings that can prevent the ingress
of moisture, dust, oils and other contaminants.

There are two
main categories: AC accelerometers, which are used with a data
collector for monitoring the condition of higher value assets,
and 4-20mA accelerometers, which are used with a PLC to measure
lower value assets. Both are capable of detecting imbalance,
bearing condition and misalignment but AC accelerometers can
also identify cavitation, looseness, gear defects and belt problems.
Hansford Sensors offers AC and 4-20mA versions of the HS-100
and HS-420 Series, which are intrinsically safe being ATEX and
IEC Ex certified. These industrial vibration sensors can be used
to monitor vibration levels on pumps, motors, fans and all other
types of rotating machinery found in marine applications.

Accelerometers
contain a piezoelectric crystal element bonded to a mass. When
the sensor is subject to an accelerative force, the mass compresses
the crystal, causing it to produce an electrical signal that
is proportional to the level of force applied. The signal is
then amplified and conditioned using inbuilt electronics that
create an output signal, which is suitable for use by higher
level data acquisition or control systems. Output data from accelerometers
mounted in key locations can either be read periodically using
sophisticated hand-held data collectors for immediate analysis
or subsequent downloading to a PC, or routed via switch boxes
to a centralised or higher level system for continuous monitoring.

The challenges
of condition monitoring at sea are great - for example, readings
taken in port will almost certainly be different from those taken
when the vessel is at sea, and heavy weather will only amplify
any such differences - so it is in everyones interest to
specify the best accelerometers and apply the best practice in
managing their performance.

To correctly
specify an accelerometer, engineers need to consider the vibration
level and frequency range that is to be measured, as well as
environmental conditions, such as the temperature and whether
there are any corrosive chemicals present. A series of further
considerations follow on from there; for example, is the atmosphere
combustible? Are there weight constraints? In marine applications,
there is no earth, which presents a further challenge but this
and other difficulties have been addressed by the designers and
engineers of accelerometers and the right research and training,
or consultation with a market leader that has experience in a
wide range of sectors, can swiftly enable the right decisions
to be made.

Once the most
appropriate sensors have been selected it is important that advice
is followed and care is taken during installation to ensure the
maximum level of performance. For example, accelerometers should
be located as close as possible to the source of vibration. Also,
devices should be mounted onto a flat, smooth, unpainted surface,
larger than the base of the accelerometer itself and this surface
should be made free from grease and oil. Condition monitoring
depends on stability and readings from a poorly mounted accelerometer
may relate not only to a change in conditions but also to the
instability of the sensor itself.

Once data has
been collected in the most appropriate and efficient manner,
machine reliability data must be analysed and interpreted, either
by on-board engineers or by a remote monitoring centre, building
a picture of machine condition and helping to create a future
maintenance schedule. With an efficient vibration monitoring
system now in place, marine engineers find they have progressed
to a new level of efficiency and, with systems well protected
by accelerometers and associated machinery to monitor vibration,
can move on to identify areas for further improvement in terms
of machine performance, energy efficiency or output. Vibration
monitoring thus becomes an increasingly powerful tool and a vital
one at a time when the effects of vibration in marine propulsion
and auxiliary systems are potentially more costly than ever.